The present disclosure relates, in general, to image sensors and, in particular, to focal plane infrared readout circuits with background suppression.
In general, image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. Imaging systems that operate in the very long infrared (IR) wavelength regionxe2x80x94in other words, in the range of about 12-18 microns (xcexcm)xe2x80x94are required for a number of space-based applications such as monitoring global atmospheric temperature profiles, relative humidity profiles, cloud characteristics and the distribution of minor constituents in the atmosphere. Such IR imaging systems also can be used for fire prevention and control, for enhanced visibility in foggy conditions, and for spectroscopic applications.
Imaging in the very long infrared wavelength region presents several design problems which primarily are a result of the presence of relatively large background signal levels. Large signal handling capacity and very small signal-to-background-contrast impose stringent requirements on the design of focal plane infrared readout circuits. A large background signal can limit the dynamic range of the imager. Furthermore, in some applications, the signal of interest may be several orders of magnitude smaller than the background signal.
For example, in one system known as the Integrated Multispectral Atmospheric Sounder (IMAS), an IR spectrometer subsystem is intended to collect data in the range of 3.74 xcexcm to about 15.4 xcexcm. Spectral data is acquired through use of a grating spectrometer and cooled quantum-well IR photodetector (QWIP) arrays. Such detectors exhibit uniform response and low 1/f noise, are reproducible at low cost, and can be manufactured in two-dimensional arrays.
To achieve high injection efficiency, however, the readout circuit for the QWIP array should have a low input impedance compared to the impedance of the photodetectors. The IMAS system has a very large background level. At its operation temperature of about 55 degrees kelvin (xc2x0K), the leakage current of the QWIPs is about 100 to 300 nano-amperes (nA), and the scene background photon-induced dark current is approximately 3.5 nA. For an integration time of 1.4 milliseconds (msec), the total dark level is on the order of 109 electrons per pixel. Therefore, the readout circuit must have a large signal handling capacity. In addition, as a result of the high detector leakage, the noise limit is governed by the shot noise in the detector dark current. For a QWIP with a photoconductive gain of approximately 0.1, the noise limit is about 10 pico-amperes (pA). In other words, the focal plane contrast, defined as the ratio of the minimum detectable signal to the background level, is very small. The readout circuit for such a system should be sufficiently accurate and sensitive to provide a differential output signal that may be as much as 85 decibels (dB) below the dark or background level.
Known techniques for handling high dynamic range include the use of switched capacitor integration, the use of gate modulated input (GMI) readout circuits, and the use of multiple gain stages. Switched capacitor integration techniques, however, require a high speed input circuit, thus limiting their feasibility. Additionally, the noise performance of switched capacitor integration techniques tends to be relatively poor. Similarly, GMI readout circuits suffer from poor noise performance because of biasing requirements and current attenuation required for obtaining a high dynamic range. The use of multiple gain stages requires prior knowledge of the dynamic range of the scene to be imaged and suffers from noise limitations in high contrast, high background environments.
In general, according to one aspect, a circuit for reading out a signal from an infrared detector includes a current-mode background-signal subtracting circuit having a current memory which can be enabled to sample and store a dark level signal from the infrared detector during a calibration phase. The signal stored by the current memory is subtracted from a signal received from the infrared detector during an imaging phase.
The readout circuit can also include a buffered direct injection input circuit and a differential voltage readout section. By performing most of the background signal estimation and subtraction in a current mode, a low gain can be provided by the buffered direct injection input circuit to keep the gain of the background signal relatively small, while a higher gain is provided by the differential voltage readout circuit.
According to another aspect, an infrared imager includes an infrared detector and a readout circuit. The imager can include an array of infrared detectors with a corresponding array of readout circuits. The readout circuit can be used with various types of detectors, including detectors that are sensitive to radiation in a range of about 12-18 microns. In one implementation, the imager includes one or more quantum well infrared photodetectors.
Various implementations include one or more of the following features. Techniques can be provided in the current memory to reduce the switch feedthrough voltage. For example, the current memory can include multiple capacitively-coupled current loops.
The readout circuit can include an integration capacitor for converting a current signal to a corresponding voltage signal that then can be sampled and stored in the differential voltage readout section. The integration capacitor can be reset.
In yet another aspect, a method of reading out a signal sensed by an infrared detector includes storing a dark level signal received from the infrared detector in a current memory circuit during a calibration phase. The signal stored in the current memory circuit is subtracted from a signal received from the infrared detector during an imaging phase. An output signal corresponding to the difference between the dark level signal received from the detector during the calibration phase and the signal received from the detector during the imaging phase can be provided.
In a related aspect, a method of reading out signals sensed by an infrared detector includes storing a dark level signal received from the infrared detector in a current memory circuit during a calibration phase. A first current signal that represents the difference between the signal stored in the current memory circuit and a signal received from the infrared detector during a first sensing period is provided. A second current signal that represents the difference between the signal stored in the current memory circuit and a signal received from the infrared detector during a second sensing period also is provided. Then, a differential voltage output signal corresponding to a difference between the first and second current signals is provided.
In some implementations, the detector is exposed to infrared radiation during the second sensing period. The first and second current signals can be converted, respectively, to first and second voltage signals. The act of converting can include integrating current signals on an integration capacitor, sampling voltages on the integration capacitor, and holding the sampled voltages on first and second capacitors, respectively.
Various implementations include one or more of the following advantages. By performing most of the background signal estimation and subtraction in a current mode, the readout circuit can operate with variable gains. In particular, a low gain can be provided by the buffered direct injection input circuit to keep the gain of the background signal relatively small, while a higher gain is provided by the differential voltage readout circuit. Therefore, the readout circuit can provide a high dynamic range and low noise performance, Furthermore, the readout circuit can be operated with low power consumption. In addition, the readout circuit can exhibit high linearity. The accurate subtraction of the background signal can enable a high effective handling capacity of more than 5xc3x97109 charges/pixel. At low operating temperatures, for example, around 55xc2x0 K, the circuit can exhibit a very small current memory leakage as well as minimal variation in the current in the memory circuit.
Other features and advantages will be readily apparent from the following description, accompanying drawings and the claims.